These contracts stop trading the Monday preceding the third Wednesday of a March quarterly cycle.
see CME Tutorial IMM stands for the International Monetary Market.
This choice of date – middle of month and middle of week – minimizes issues with date rolling, as holidays are very unlikely to make the closest business day in another week or other month.
From late 2002, the CDS market began to standardize credit default swap contracts so that they would all mature on one of the four days of 20 March, 20 June, 20 September and 20 December.
[2][3] Contracts are frequently rolled on the IMM dates, making them among the highest volume trading days of the year.